Description
The XPS foundation board 90mm is a blue, water-repellent board, 1000 × 500 mm, sold one board at a time from the Renders World XPS foundation board range. It insulates the foundation and the plinth of a wall whose facade above the DPC is 90 mm thick.
Where the XPS Foundation Board 90mm Goes
A rendered wall is insulated in two parts. Above the damp-proof course a facade board carries the render; below it the board has to cope with splash water, wet soil and backfill. This board is the lower part, cut to the same 90 mm depth as the facade so the two meet in one plane:
- Foundation walls below ground, bonded to the wall before the trench is backfilled.
- The plinth from ground level up to the DPC, the strip that takes rain splash and garden soil.
- The perimeter of the foundation, so the insulation runs on down past the plinth without a gap.
Why Choose the 90 mm Depth
- It lines up with a 90 mm facade. Mesh and render cross the DPC line flat, with no step to pack out.
- About 2.35 m²K/W of thermal resistance: 0.090 ÷ 0.038 = 2.37, rounded down to 2.35.
- 100 kPa compressive strength, so the board holds its shape under backfill and paving set against the plinth.
- 5 % water absorption under long-term immersion, with 150 kPa bending strength, both from the CE marking on the board.
Specification — XPS Foundation Board 90mm
| Property | Value |
|---|---|
| Board | XPS foundation board, blue, water-repellent |
| Thickness | 90 mm |
| Board size | 1000 × 500 mm (0.5 m²) |
| Sold as | 1 board |
| Thermal conductivity λD | 0.038 W/mK |
| Thermal resistance | 2.35 m²K/W (0.090 ÷ 0.038 = 2.37, rounded down) |
| Compressive strength | 100 kPa |
| Bending strength | 150 kPa |
| Water absorption, long-term immersion | 5 % |
| Reaction to fire (the board alone) | Class E |
What Goes With the 90 mm Board
Above the DPC, carry on with the 90 mm graphite EPS facade board at the same depth. The boards are bonded with an adhesive from the adhesives and basecoats for insulation boards; the starter track and anchors for the facade come from the insulation fixing accessories; and the visible plinth is usually finished in a mosaic plinth render, which stands up to splash water. How the two zones join at the DPC without a cold line is shown in the plinth thermal bridge guide.
The 90 mm Board in the Range
| Board | Thermal resistance (thickness ÷ 0.038) | Typical place |
|---|---|---|
| 30 mm | 0.75 m²K/W | Thin plinths where little depth is free |
| 50 mm | 1.30 m²K/W | Shallow plinths and foundation wraps |
| 80 mm | 2.10 m²K/W | Plinths under an 80 mm facade |
| 90 mm (this board) | 2.35 m²K/W | Plinths under a 90 mm facade |
| 100 mm | 2.60 m²K/W | Deepest wrap, 100 mm facades |
All five depths share λD 0.038 W/mK, so the only choice is depth. What 90 mm does for the whole wall is worked through in the U-value and insulation thickness guide.
How Many 90 mm Boards
One XPS foundation board 90mm covers 0.5 m², so two make a square metre, and Renders World sells it singly — no full pack to buy. Measure the length of the plinth and the depth you will insulate, multiply them, double the result and round up. Add a few spare boards for corners, steps in the ground and cuts around pipes.
FAQ — XPS Foundation Board 90mm
What size is one board, and how much does it cover?
Each XPS foundation board 90mm is 1000 × 500 mm. It covers 0.5 m², so you need two boards for every square metre of foundation or plinth.
Can I take it up the wall above the DPC?
Keep it to the foundation and the plinth. Above the DPC the wall takes a facade board made for render — the 90 mm graphite board keeps the same depth, so the render runs on without a step.
90 mm or 100 mm?
Match the facade above. 90 mm gives about 2.35 m²K/W and 100 mm about 2.60; where the facade is 100 mm thick, take the 100 mm foundation board.
What does fire class E mean on a rendered plinth?
Class E is the board on its own. A finished, rendered plinth is judged as a whole system, so the wall's class comes from that system, not from this board.
Which adhesive holds the 90 mm board below ground?
Use a board adhesive from the adhesives and basecoats range whose own sheet lists XPS boards and damp conditions. Then cover the board before the trench is backfilled, so the soil does not bear straight on it.